High-resolution, high-speed inline optical topography measurement system for laser micro-machining process control

An in situ topography measurement system for monitoring and controlling laser micromachining processes is presented in this paper. A frequency-domain low-coherence interferometry (FD-LCI) system is used to generate high-resolution, high-speed topographic data that will enable an automatically adaptive micro-machining process with increased machining precision. To provide the topographic data directly in machine coordinates, the measurement beam has to be co-axial with the ultra-short pulse machining laser (wavelength 515 nm). A camera sensor-based module was developed to measure offset and angle between both beams, which can be corrected by fine-adjustment of the measurement beam. The reflectivity of the workpiece can vary over a very wide range. To maintain an optimal strength of the interferometry signal while scanning the workpiece surface, the feedback light power from the reference path is adapted with a high-speed beam shutter, such that it always matches the return light levels from the measurement path. Preliminary tests indicate that the FD-OCT system can provide topographic measurement points at varying rates of up to 20 kHz, with a depth measurement range of zrange = 1 mm and a depth resolution better than 4 mm.